Make gtk_print_dialog_setup_finish return a GtkPrintSetup
object, which encapsulates all the data that needs to be
transferred between the setup and print calls, and make
the print_file and print methods take an extra GtkPrintSetup
argument.

Change the print call to return an output stream, rather
than take an input stream. The results are now returned
when the output stream is closed.

With some further cleanup, this makes the GtkPrintDialog
object a proper builder object - you can create multiple
print dialogs from the same GtkPrintDialog object, in
parallel, and they won't interfere with each other.